Not Applicable
Not Applicable
1. Field of the Invention
This invention is in the field of devices used in gas lift operations in oil wells.
2. Background Art
An oil well is drilled into a hydrocarbon bearing earth formation, where the well is typically xe2x80x9ccompletedxe2x80x9d to allow production of hydrocarbon material from the formation. Hydrocarbon production often begins with sufficient gas pressure in the formation to force the oil to the surface. As production from the well continues, the reservoir usually loses pressure until production of oil from the well is no longer provided by the formation gas. Sometimes, the formation pressure is insufficient to support production, even when the well is first completed.
In either case, it is common to modify a well to allow the injection of pressurized gas from the surface, to supplement the formation gas in lifting the well fluids to the surface. This is commonly called a xe2x80x9cgas liftxe2x80x9d operation. More specifically, high pressure gas from the surface may be applied to the annulus of the well surrounding the production tubing. This gas enters the production tubing from the annulus, through a gas lift valving mechanism which is commonly positioned in a side pocket or bore, commonly called a valve pocket, within a mandrel. Passages are commonly provided for the gas into the valve pocket, through the mandrel wall from the annulus. The valve in the valve pocket then controls the actual flow of gas according to its specific design. The mandrel body, sometimes called a xe2x80x9cvalve bodyxe2x80x9d, is also typically equipped with another passage, or through-bore, which goes straight through the valve body and on down the production tubing.
When the gas enters the production tubing via the mandrel, it can be used to create a venturi effect and draw well fluids into the production tubing. The gas can also entrain itself into the well fluids, thereby lowering the specific gravity of the fluid and assisting in removal of the fluid from the well. A similar mandrel can be used for water or chemical injection into the well, through the tubing, from the surface.
The valve which actually controls gas flow is typically lowered through the production tubing by wireline and guided into the valve pocket, such as with a tool commonly called a xe2x80x9ckickover toolxe2x80x9d. This allows placement of the valve pocket to one side of the mandrel body, parallel to, but laterally offset from the through-bore, and entirely out of the through-bore. That is, the through-bore commonly runs straight from one production tubing connection, alongside the valve pocket, to a second production tubing connection. This parallel but offset arrangement is facilitated by the use of transitional end caps or xe2x80x9cswagesxe2x80x9d on the ends of the mandrel body. The end caps are referred to as xe2x80x9ctransitionalxe2x80x9d pieces herein, because they transition in diameter from small to large, on the uphole end of the mandrel body, and from large back to small, on the downhole end of the mandrel body. Typically, that is, each end cap has a large end which matches the diameter of the valve body, and a small end which matches the diameter of the production tubing. The small end is offset completely against one side of the end cap, relative to the large end. In fact, the wall of the small end can align with the wall of the large end, and the two ends can have identical wall thicknesses. So, when installed, the large end aligns with the valve body, while the small end aligns with the through-bore in the valve body. This results in straight-through flow of production fluid, while generating minimal back pressure.
It is desirable to have a through-bore in the mandrel which has a xe2x80x9cfull borexe2x80x9d diameter, that is, where the inner diameter of the through-bore all the way through the mandrel body is at least as large as the inner diameter of the production tubing to which the small ends of the mandrel end caps are connected. One reason for this is that it is economically very important to maintain the inner diameter of the fluid production passage as large as possible, relative to the overall diameter of the mandrel. Another way to state this is that it is very important to minimize the overall diameter of the mandrel relative to the inner diameter of the through-bore. Put either way, the point is to be able to install as small a mandrel as possible, with a through-bore as large as possible, to maximize the rate of production of fluid from a given diameter of well casing.
Known gas lift mandrels have most often had the transitional end caps welded to the valve body, or they have been one-piece mandrels, cast or machined with integral end caps. Welded mandrels have high manufacturing costs, and they tend to be less uniform than desired, while one-piece mandrels have high tooling costs, and high capital equipment costs. In the past, attempts to thread the end caps onto the valve body have failed, because the thread designs utilized were thicker than the wall thickness of the components they joined, and because o-rings were required to achieve fluid tight seals. Bulky thread sets, with wall thickness thicker than the joined components, had the distinct disadvantage of increasing the overall diameter of the mandrel assembly, and decreasing the diameter of the through-bore flow path, at least where it passed through the thread sets. This resulted in the use of a smaller mandrel body, and a smaller through-bore diameter, in a given size of casing. Mandrels and other tubular components sealed with o-rings have not been favored, because of the tendency to lose the seal under harsh downhole conditions.
Therefore, it would be desirable to have a gas lift mandrel which operates exactly the same as mandrels with which operators are familiar, but which have separate end caps joined to the mandrel body by some process other than welding, where the resulting mandrel assembly has as large a through-bore diameter as possible, and as small an overall diameter as possible, and where the end caps reliably maintain their seals for the life of the mandrel.
The present invention provides a side-pocket type gas lift mandrel in which the transitional end caps are threaded to the mandrel body. The threads used on each component have a thickness no greater than the wall thickness of the component itself. Further, when male and female threads are threaded together, they create a thread set which has a thickness no greater than the wall thickness of either of the two components joined thereby. When the end caps are threaded to the mandrel body with these threads, the overall diameter of the assembly, at the locations of the thread sets, is no greater than the overall diameter of the mandrel body itself. Also, since the thickness of the assembled thread set is no greater than the wall thickness of the end cap, there is no reduction in the inside diameter of the through-bore as it passes through the thread set.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: